Worming Horses: Advice from an Expert

Horse Worming

Worming: Expert advice on worming horses

Worming horses has been a hotly debated topic for many years. The need to worm, quantity and type of wormer has been a recurring conversation delivered by many different chanels following expert advice. But what are the experts actually saying?

In this article, Rosie Naylor BVetMed MVetMed DipACVIM PhD MRCVS details several important areas associated to worming horses by covering topics such as resistance, treatments, assessing need and seasonal worming.

With thanks to Rosie Naylor BVetMed MVetMed DipACVIM PhD MRCVS

Resistance to wormers is a growing problem

With increasing reports of resistance to most wormers, if not all, there is now an important need to weigh up the risk of disease and the need for treatment in an individual horse with the health of the broader equine population.


Strategies should be designed to identify and treat only those animals with worm burdens that are potentially detrimental to their health and that are shedding eggs and contaminating the environment of other animals. This can be done by performing worm counts to identify those that are not suffering with a heavy burden.

Effective ways to reduce the need to use wormer treatments

The need for wormer treatment will be significantly reduced if the environmental worm challenge can be reduced.

Husbandry practices, therefore, have an important part to play in limiting the use of wormer products. The aim should be to minimise stocking density to ensure at least 1 acre of grazing per horse and to maintain consistent horse populations, using faecal egg counts (FECs) to monitor environmental contamination.

Droppings should be removed from the pasture at least twice per week and muck heaps kept far away from the paddocks. Resting and rotating paddocks is best done in the summer months as the larvae will survive for less time in hot dry weather. Co-grazing with sheep will also help to reduce the infective burden on the pasture.

worm horses - image of horse in field

Quarantine protocols are an important way of preventing new horses bringing worms onto the yard. Horses should be treated with a broad spectrum wormer and ideally a FEC would be performed before and 2 weeks after treatment, to confirm this has been effective, before turning the horse out. In practice, treatment with moxidectin and praziquantel is usually administered followed by stabling the horse for a minimum of 3 days after treatment.

Assessing the need for wormer treatment

It is well accepted that interval dosing every horse without assessing the need for treatment is obsolete. Faecal egg counts should be performed to identify the 20% of horses that shed 80% of the worm eggs and to whom treatments should be targeted. The frequency of FECs will be determined by the level of risk but in most cases, three FECs should be performed between March and September. Such targeted worming strategies have been shown to reduce the cost of worming. Results of >200-250 eggs per gram of faeces usually warrant treatment.

Repeating FECs 10-14 days after treatment and comparing pre and post-treatment results is the principle of the faecal egg count reduction test. This is an important way of monitoring the effectiveness of treatments administered and identify if any resistance is present.

Exposure to tapeworm will not be detected by FECs and diagnosis of tapeworm exposure requires a serum antibody or saliva test. Specific testing for tapeworm is recommended every 6-12 months to determine the need for treatment.

What to use when worming is needed

Autumn de-worming

At the end of the grazing season, worming is recommended to reduce the risk of larval cyathostominosis. In moderate to high risk animals, treatment with moxidectin is prudent. In low risk animals that have had repeatedly negative faecal egg counts throughout the grazing season this may be unnecessary or ivermectin may be a suitable alternative thereby reducing moxidectin exposure. If the level of risk is unknown moxidectin should be administered as a precaution.

During the spring and summer

During the grazing season, the panel recommends the use of pyrantel or ivermectin in horses with a high faecal egg count. It may be logical to rotate between the two. As moxidectin is the most effective option in treating cyathostominosis the authors concluded that it should be preserved and its prophylactic use reduced.

On well managed properties, dosing and moving horses after treatment may reduce the number of worms in refugia and actually increase the selection pressure for resistance, therefore such practice is not advised. On poorly managed properties however, where the risk of reinfection is high, this approach may still be necessary.


If tapeworms are being targeted specifically, praziquantel is the treatment of choice. In the absence of a product containing only praziquantel, the authors suggest that ivermectin combinations should be used where possible when worming horses, to reduce unnecessary use of moxidectin. If moxidectin treatment is also required, a combination product is available. If treatment of strongyles is needed, a double dose of pyrantel also targets both strongyles and tapeworm and is an alternative to combination products.

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